The XML has been defined as a superset of the SGML5.5 which is the superset of the HTML. The XML is an application profile or restricted form of the SGML. By construction, XML documents are conforming SGML documents.
An XML document consists of several sections or objects called tags. Each tag looks like:
It means that here starts a tag which is named 
. The most of
tags have theirs ending tags as well. They look like:
Everything between a starting tag and an ending tag is treated as data which are described by the mentioned tag.
Usually XML documents are text files but they could be also partially binary if appropriate tags are defined.
What is the most important, tags could be recursive (if the specified grammar allows, of course). And this feature is used by the Osiris too.
XML documents may, and should, begin with an XML declaration tag which specifies the version of XML being used. For example, the following is a complete well-formed XML document:
<?xml version="1.0"?> <greeting>Hello, world!</greeting>
The XML is also called a metalanguage, because it doesn't define a language but it gives certain rules how to create a language which uses tags to define its grammar.